Eight-hundredy Billion ago... DINOSAURS!

Time passes, the fight ensues. Not everyone is content to beat Robotnik by just jumping on his shit and collecting rings. Utilizing recovered weapons and supplies small militias and guerrilla squads are formed. Mycroft was not a fighter, he was content to spend his time carefully studying and cataloging history records. But when he heard that Harley was going to join one of the front line forces... he stepped in and took the last spot before she got the chance. They gave him a big gun and she thinks his beret is cute. It is heavy. The gun, not the beret.

He spends some free time training Harley as a sniper so that when he finally can't stop her from entering the fight she'll at least be fairly far from the front lines... in a sense.